Efficient phase shift migration for ultrasonic full-matrix imaging of multilayer composite structures

Abstract

Multilayer composite structures are essential for various fields but void defects inside will greatly reduce its mechanical properties. Ultrasonic full-matrix capture (FMC) is a useful imaging modality to detect defects in multilayer structures, but it also brings heavy computational burden. Though great progress has been made in the full-matrix imaging for homogeneous medium, most of these methods are invalid or inefficient for the cases of multilayer structures. To amend it, an efficient phase shift migration for ultrasonic full-matrix imaging of multilayer structures is proposed in this paper. The method regards FMC as data in three dimensions: receiver position, transmitter position and time, and transforms it into frequency-wavenumber (f-k) space by a three-dimensional fast Fourier transform. By downward extrapolating the wavefield on the surface, the wavefield at arbitrary depth in the measuring region is reconstructed, with a firstly introduced double-square-root vertical wavenumber. Then the imaging condition t = 0 is directly applied in the f-k space to produce the defect image. Computational complexity was reduced greatly, and the calculation time of the proposed method was only 1/6 of that of the well-established extended phase shift migration, in the case of the same resolution. Therefore, the proposed method has great potential in on-line ultrasound defect detection, especially for cases of large multilayer structures.